1. Technical Field of the Invention
The present invention relates to digital downconversion and, more particularly, to a method and apparatus for multiphase component downconversion.
2. Description of the Related Art
In a radio transmission system a transmitter generates digital symbols from digital data and transmits such symbols for the benefit of a receiver. The channel can be wireless or wired. If the channel is a radio frequency (RF) wireless channel, time dispersion can be introduced into a signal before reception at the receiver. Fading, cochanel and adjacent channel interference and noise can also be introduced into the signal.
A transmitter generates an output such as digital symbols S(n). The received signal is filtered and sampled to produce a received digital signal y(n) which is sent to a demodulator (e.g. a channel equalizer). For any type of signal modulation (e.g. FM, QPSK, OQPSK, .pi./4-DQPSK, GMSK, DS-CDMA) where a quadrature representation of the signal is desired, at the receiver it is necessary to provide for quadrature downconversion and elimination of signal impairments caused by the receiver and the channel.
A TDMA (Time Division Multiple Access) radio transmission is a time-shared transmission on separate timeslots 1 to N. A TDMA radio transmission can be on a single frequency carrier. A different signal sequence SS, which includes a synchronizing sequence SO and a data sequence DO with the information to be transmitted, can be transmitted in each timeslot. The signal sequence SS contains a binary signal, although the aforesaid symbols S(n) can be coded according, for instance, to the QPSK-code. In a complex number plane, with axes designated I and Q, four possible values of the symbols S(n) are marked one in each quadrant with the binary numbers 00, 01, 10, or 11.
A spread spectrum DS-CDMA (Direct Sequence - Code Division Multiple Access) radio transmission system transmits to all users at the same time and on the same channel frequency by spreading each user's signal sequence SSi with a pseudonoise (PN) sequence PNi* (*denotes complex conjugate). Each user's signal sequence is spread in such a manner that each signal can be uniquely despread at the receiver by using the corresponding synchronous PN sequence, PNi, while at the same time reducing the other users signals (interference) so that the signal quality is not impaired. In addition to, or in place of the PN sequences, a signaling set consisting of Walsh sequences can be used to further discriminate between user's signals (spread and despread) as is described, for example, in TIA/EIA IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.
In a receiver there are many ways that a direct current (DC) or carrier error term can be introduced in the desired signal. In the baseband circuitry a DC term can be introduced due to operational amplifier offsets, demodulator offset voltages, and/or analog-to-digital converter characteristics. Likewise, since a coherent local oscillator (L.O.) is required for demodulating the desired signal to baseband, there can be self-demodulation of any local oscillator leakage which would then produce a baseband DC error term. If the received signal is converted to an intermediate frequency (IF) and then digitized, a similar phenomena can introduce errors especially when the local oscillator's frequency or intermediate frequencies are chosen to be either coherent or related to the bit rate (often done to simplify the sampling and processing of the signal). For most systems the DC error term (as well as any DC term which is part of the desired signal) can be eliminated by alternating current (A.C.) coupling with a cut-off frequency sufficiently low so as not to appreciably effect the content of the desired signal. However, if the receiver is automatic gain controlled, then the A.C. coupling will most likely not prove effective because the DC offset will be dynamic and most likely at a rate above the A.C. coupling cut-off frequency. This is the case in a TDMA system. In a code division multiple access (CDMA) system the DC offsets can be especially troublesome since the baseband signal has inherent DC terms which must be present to successfully demodulate the signal where the addition of other DC terms would greatly degrade demodulation performance.
Automatic gain control (AGC) can be used in a receiver to limit the required dynamic range of a receiver. Among other things, automatic gain control will limit a number of required analog-to-digital converter bits. Usually, in a TDMA system, it is desired to track signal attenuation due to lognormal fading (shadowing) rather than Rayleigh fading (fast fading). In a CDMA system, automatic gain control (AGC) sets the variance of the resultant white noise resembling composite signal as seen by the analog-to-digital converter.
Receivers typically perform quadrature downconversion upon received signals modulated in a complex plane. Quadrature downconversion can be performed in analog receivers or digital receivers such as TDMA and CDMA receivers. Quadrature downconversion from an intermediate frequency (IF) is conventionally performed, for example, in an analog receiver, by inputting a passband analog signal into two analog mixers in parallel followed by lowpass filters to eliminate double frequency signal components. Analog-to-digital converters are used to sample the resultant analog in-phase and quadrature baseband signals.
Various types of digital quadrature downconverters can also be implemented. One advantage of these digital downconverters over the analog downconverter is a reduction in the number of analog-to-digital converters required. A first type of digital downconverter, for example, requires a high speed analog-to-digital converter, followed by two digital mixers and decimation filters. Such an implementation is provided, for example, in Harris Part No. HSP 50016. A second type of digital downconverter also uses one analog-to-digital converter. The sampling frequency (fs) and final intermediate frequency (fIF) are chosen such that the samples of the signal need to be alternatingly fed with corresponding sign changes to two digital lowpass interpolating filters to obtain the desired in-phase and quadrature baseband signals. Such a digital downconverter is provided, for example, in Harris Part No. 43216 or by L. E. Pellon, "A Double Nyquist Digital Product Detector for Quadrature Sampling", IEEE Transactions on Signal Processing, July 1992, pp. 1670-1681.
Another type of digital downconversion uses a discrete-time Hilbert filter together with a complex downconverter state. Such a type of Hilbert downconverter is described in "Quadrature sampling with high dynamic range", IEEE Transactions Aerospace Electronic Systems, vol. AE8-18, no. 4, pp. 736-739, November 1982, which is incorporated herein by reference. Such a type of Hilbert downconverter also uses one less analog-to-digital converter than the analog downconverter. One less analog-to-digital converter allows one less receiver branch providing better gain balance between the in-phase and quadrature signal. The phase accuracy and gain balance of a Hilbert downconverter between the in-phase and quadrature signal components is also more accurate and not sensitive to temperature variations or aging (component drifting).
The above-mentioned first digital downconverter requires a much higher speed analog-to-digital converter than the analog downconverter. Also, the first digital downconverter requires actual high speed multiplication for mixing down the signal to extract the in-phase and quadrature baseband signals. The second digital downconverter does not require a high speed analog-to-digital converter or high speed multiplication because it uses a multiplierless final downconversion stage. The second digital approach, however, requires expensive A/D converters because of the required sampling rate. The Hilbert downconverter requires a complex mixing stage to perform downconversion. Expensive hardware or extensive processor time must be used to multiply the complex numbers.